1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, to a pneumatic tire suitable for a high performance vehicle in which high wet-condition drainage performance, controlling stability and tire wear resistance can be improved without sacrificing other tire properties.
2. Description of the Related Art
Conventionally, in order to suppress the occurrence of hydroplaning in a tire having a wide tread width, such as rear tires on super high-performance vehicles for racing competitions, methods have been employed which set a plurality of straight, main grooves in a central region of the tire, or which increase the negative ratio.
However, problems such as grip deterioration and deterioration in tire wear resistance arise because the area of land portions in the central region of the tire and also block width decrease.
Taking these facts into consideration, an object of the present invention is to provide a pneumatic tire suitable for a high performance vehicle, in which high wet-condition drainage performance, controlling stability, and tire wear resistance can be improved without sacrificing other tire properties. A first aspect of the present invention is a pneumatic tire for mounting to a wheel and supporting a vehicle above a surface, the tire comprising a tread having opposite edges and a surface contacting portion, and an equatorial centerline plane, the tread including: (a) at least three circumferential main grooves extending through a central area in the surface contacting portion, the central area being located within 45% of one-half of a width of the surface contacting portion from the equatorial centerline plane along an axial direction of the tire towards the edges of the tread, each circumferential main groove having a groove width from 6% to 10% of one-half of said width of the surface contacting portion, with groove center lines of two tire-axial-direction outermost circumferential main grooves being located distanced from 30% to 40% of one-half of said width from the equatorial centerline plane towards each of the edges of the tread; (b) a plurality of slanted lateral main grooves, each having opposite ends, one end of each slanted lateral main groove opening into a circumferential main groove and the other end of each slanted lateral main groove opening into one of the edges of the tread, each slanted lateral main groove slanting such that said one end thereof is positioned further along a direction of rotation of the tire relative to said other end of that slanted lateral main groove when the tire is rolling forwardly as used on a vehicle, each slanted lateral main groove having a groove width in a range of 8 mm to 11 mm, a tire circumferential direction spacing from one another in a range of 45 mm to 52 mm, an angle with respect to the tire circumferential direction, between two tire-axial-direction outermost circumferential main grooves, being within a range of 20xc2x0 to 60xc2x0, and an angle with respect to the tire circumferential direction, outside the two tire-axial-direction outermost circumferential main grooves, being within a range of 45xc2x0 to 90xc2x0; and (c) a plurality of land portions defined by the adjacent circumferential main grooves, with some land portions nearer to the equatorial centerline plane than any of the other land portions, said some land portions having a tire axial direction width of 6% to 10% of the surface contact width.
Generally, when a tire having a wide tread width (e.g., a tread width of 250 mm or greater), such as rear tires on super high-performance vehicles for racing competitions, is run at a high speed on a wet road surface, water on the road surface is drained in the direction that the tire advances, at a region extending in the outboard side direction from the inboard side tread edge to a position approximately 45% of a ground contact half width distanced from a tire equatorial plane.
In such a case as this, a pneumatic tire according to the present aspect achieves the following excellent effects.
Firstly, according to a pneumatic tire of the present aspect, as at least three circumferential main grooves extending along the tire circumferential direction are provided in a central area extending in the outboard/inboard direction from a tire equatorial plane to a position 45% of a ground contact half width distanced from the tire equatorial plane, drainage from the central region of the tire can be achieved efficiently.
Further, because each circumferential main groove has a groove width from 6% to 10% of one-half of said width of the surface contacting portion, and groove center lines of two tire-axial-direction outermost circumferential main grooves are located distanced from 30% to 40% of one-half of said width from the equatorial centerline plane towards each of the edges of the tread, drainage from the tire central area becomes most efficient.
The number of circumferential main grooves provided in the central area is preferably four.
When the tire axial direction width of land portions nearest the tire equatorial plane is less than 6% of the ground contact width, the width of the land portions becomes too narrow, and wear resistance and controlling stability deteriorates.
On the other hand, when the width of the same land portions exceeds 10% of the ground contact width, problems such as impairment of ground contact properties during acceleration, and insufficient traction and unstable behavior at the time cornering is initiated are generated.
When the tire axial direction width of the same land portions is less than 6% of the ground contact width, drainage performance drops and performance in wet conditions declines. When the tire axial direction width of land portions exceeds 10% of the ground contact width, land portion areas decrease and resistance to tire wear and controlling stability deteriorate.
When the groove width of the slanted lateral main grooves is less than 8 mm, drainage performance declines, and when the same groove width exceeds 11 mm, land portion areas decrease and resistance to tire wear and controlling stability deteriorate.
When the tire circumferential direction spacing of the slanted lateral main grooves is less than 45 mm, land portion areas decrease and resistance to tire wear and controlling stability deteriorate. When the spacing exceeds 52 mm, drainage performance declines.
When the angle formed by the slanted lateral main groove with respect to the tire circumferential direction between the two tire-axial-direction outermost circumferential main grooves falls outside the range described above, drainage performance declines.
In a second aspect of the present invention, the tread has a negative ratio within a range of 45% to 50% at the central area, a negative ratio of 40% to 43% at an outboard-side area which is adjacent to the central area at the outboard side, and a negative ratio of 40% to 45% at an inboard-side area which is adjacent to the central area at the inboard side.
In this manner, when a negative ratio of the tread is within a range of 45% to 50% at the central area, within a range of 40% to 43% at the outboard-side area, and within a range of 40% to 45% at the inboard-side area, balanced improvement of drainage performance in wet conditions, controlling stability and tire wear resistance can be altogether reliably secured.
In a third aspect of the present invention, the pneumatic tire having the first and second aspects is adapted for use as a rear tire of a vehicle.
According to the present aspect, by using for the rear tires of a vehicle a pneumatic tire having the first and second aspects described above, the effects resulting from the working of the present invention can be most effectively demonstrated.
In a fourth aspect of the present invention, the pneumatic tire having the above aspects is adapted for use on a vehicle with wheel alignment settings such that a toe angle is in a toe-in range of from about 0xc2x0 to about 0.7xc2x0 and a camber angle is in a negative camber range of from about 0xc2x0 to about 2xc2x0.
According to the present aspect, when the pneumatic tire having the above characteristics is mounted on a vehicle having wheel alignment settings such that a toe angle is in a toe-in range of from about 0xc2x0 to about 0.7xc2x0 and a camber angle is in a negative camber range of from about 0xc2x0 to about 2xc2x0, effects resulting from the working of the present invention can be most optimally demonstrated.